The present invention relates to the structure of a protection circuit for a semiconductor device, and more particularly, it relates to an overcurrent protection circuit, used in a semiconductor device including an insulated gate switching element, for protecting the switching element from an overcurrent.
FIG. 7 shows the general cross-sectional structure of a lateral insulated gate bipolar transistor (hereinafter referred to as the IGBT). In the lateral IGBT 51 shown in FIG. 7, a P−-type base region 205 is formed in a surface portion of an N−-type semiconductor substrate 201. An N+-type emitter region 206 is formed in a surface portion of the base region 205. Also, a gate insulating film 203 is formed over the base region 205 so as to extend from a portion on the emitter region 206 to a portion on the semiconductor substrate 201, and a gate electrode 204 is formed on the gate insulating film 203. Furthermore, a P-type collector region 202 is formed in another surface portion of the semiconductor substrate 201 to be spaced from the base region 205.
Moreover, a collector terminal P1′ electrically connected to the collector region 202, a gate terminal P2′ electrically connected to the gate electrode 204 and an emitter terminal P3′ electrically connected to the emitter region 206 are formed above the semiconductor substrate 201.
The lateral IGBT 51 of FIG. 7 is turned on when a forward bias voltage is applied between the gate terminal P2′ and the emitter terminal P3′ with a side of the collector terminal P1′ set to high potential. Alternatively, it is turned off when a zero bias voltage or a backward bias voltage is applied between the gate terminal P2′ and the emitter terminal P3′. Thus, the lateral IGBT 51 has a switching characteristic that it is turned on/off in accordance with a gate voltage applied to the gate electrode 204.
A semiconductor device including such a lateral IGBT 51 is occasionally used with an inductive load connected between the collector terminal P1′ of the lateral IGBT 51 and a power supply. If an accident happens in such a case, the inductive load is short-circuited, and hence, a current not less than several times of rated current passes through the lateral IGBT 51. When the load is thus short-circuited, such an overcurrent should be detected so as to interrupt the gate voltage or the collector voltage because otherwise the lateral IGBT 51 may be thermally broken through temperature increase.
Therefore, a semiconductor device having an overcurrent protection function for the lateral IGBT 51 shown in FIG. 8 (see, for example, Japanese Laid-Open Patent Publication No. 09-260592) has been proposed. The semiconductor device 50 of FIG. 8 includes the lateral IGBT 51 as a principal switching element controllable in accordance with a gate voltage, and further includes a current detecting lateral IGBT 52 connected to the lateral IGBT 51 in parallel. At this point, an emitter region of the current detecting lateral IGBT 52 is electrically connected to a sense resistor 54 working as a current detecting resistor. Also, a current detecting circuit 57 electrically connected to the current detecting lateral IGBT 52 includes a voltage comparator 56, and a reference voltage circuit 55 and the sense resistor 54 both connected to the voltage comparator 56.
In the semiconductor device 50 of FIG. 8, a current 59 passing through the current detecting lateral IGBT 52 flows through the sense resistor 54 to the emitter terminal P3′. At this point, a voltage generated between both ends of the sense resistor 54 and a voltage generated by the reference voltage circuit 55 are compared with each other by the voltage comparator 56, so that the current 58 passing through the lateral IGBT 51 corresponding to the principal switching element can be controlled on the basis of the thus obtained voltage difference.